Redescriptions of Neanthes Kinberg, 1865 (Annelida: Errantia: Nereididae) species from worldwide regions

The present study redescribes four species of Neanthes Kinberg, 1865 (Nereididae de Blainville, 1818) based on their type specimens collected from different worldwide localities: Neanthes chilkaensis (Southern, 1921) from India, N. galetae (Fauchald, 1977) from Panama, N. helenae (Kinberg, 1865) from St Helena Island, and N. mossambica (Day, 1957) from Mozambique. The morphology of the types was re-examined for the first time after the species were originally described, and incorporated the recent improvements in the standards and terminology for describing nereidid features. The arrangement of paragnaths on area VI stood out among the diagnostic features used to distinguish these four species. Neanthes chilkaensis and N. helenae are the unique nereidids bearing p-bar paragnaths on the area VI. Both species are also distinctive as the former species only exhibited p-bar paragnaths on the area VII– VIII and the latter ventrolateral projections on the apodous segment. Further examination revealed that N. nanciae (Day, 1949) from St Helena is a junior synonym of N. helenae. Moreover, N. galetae and N. mossambica are distinguishable from other species also by the development of dorsal cirri, neuropodial postchaetal lobe and ventral ligule, the presence/absence of merged paragnaths on area IV, paired oesophageal caeca, among other features. This study has further contributed to the morphological delimitation of the species in Neanthes as a first step towards revising the genus.

In the present study, four species that are currently placed in Neanthes are redescribed based on type material from isolated localities in India, Panama, Mozambique and St Helena Island. This study aims to provide more morphological delimitations of the species within the genus Neanthes.

Material and methods
The type materials examined in this study are deposited in the following museums. The total length (TL), length from the distal end of prostomium to chaetiger 15 (L15), and body width at chaetiger 15 excluding parapodia (W15) were measured; and the total number of chaetigers was counted for complete specimens. Denticles on the jaws and paragnaths on unpaired and paired sides (denoted as 'a' for left and 'b' for right) of pharyngeal areas were counted (range: min-max). Features of non-everted pharynx were observed by conducting a longitudinal dissection in the mid-ventral oral region. Parapodia were dissected and mounted on glass slides to examine parapodial features. Decimal numbers are used for practical purposes when measurements between two structures exceeded one unit (e.g., 1.2 times, 2.5 times, twice); whereas written fractions were used when those measurements were less than one unit (e.g., half, two-thirds, four-fifths).
Light microscopy observations were carried out using both stereo and compound microscopes. Specimens were photographed using a digital camera (Nikon D5100) mounted on the eyepiece of each microscope with a portable microscope adaptor. Around 15-20 photos were stacked to improve the visual depth of field using Helicon Focus ® 6 (Method C). The figure backgrounds were cleaned and darkened or lightened as necessary, and the final figures assembled in plates using Adobe Photoshop ® CS6.
Descriptions of the species are based upon one single primary type morphology unless otherwise stated. Variations for the remaining type material are indicated in parentheses. The relative extension of parapodial structures and the relative width of ligules and lobes were described following recent studies (Villalobos-Guerrero & Carrera-Parra 2015; Conde-Vela 2018). However, the dorsal cirri were measured considering the length of the proximal region of dorsal ligules, whereas the distal region of dorsal ligules was measured with regard to the length of the proximal region of dorsal ligules (Villalobos-Guerrero 2019).
The nereidid parapodial terminology and standardised definitions of the articulations of chaetae by Villalobos-Guerrero & Bakken (2018) were followed. The size of falciger blades (B/A ratio) and the length of its serrated edge concerning the total blade length were described following Bakken & Wilson (2005) and Glasby & Hsieh (2006), respectively. The paragnaths terminology of Bakken et al. (2009), partially readapted by Conde-Vela (2018), and the characterisation of the jaws by Jansonius & Craig (1971), were used. Finally, the description of the pharynx arrangement at the dorsal areas of the oral ring, namely 'area VI-V-VI ridge pattern', is based on Villalobos-Guerrero (2019).

Description
Colour and measurements. Lectotype atokous, complete, in good condition, 43 mm TL, 12.2 mm L15, 2 mm W15, with 74 chaetigers. Body colour brownish, with brown pigmentation in dorsum of prostomium and palps, and present throughout in dorsum and venter of segments ( Fig. 1A-B).
PalPoPhores. Sub-conical, as wide as long (Fig. 1B), as long as three-fifths of entire prostomium; with distinct sub-distal transverse groove. Palpostyles ovoid, thick, with diameter as wide as one-third of palpophore.
antennae. Tapered, slender, long, extending forwards beyond tip of palpophore and posteriorly to halflength of prostomium; antennae well separated, with gap 1.2 times as wide as basal diameter of antennae.
eyes. Paired eyes blackish, arranged in a trapezoid form; gap between both pairs two-fifths as wide as diameter of posterior pair of eyes (Fig. 1B); anterior pair of eyes rounded, 1.5 times as wide as basal diameter of antennae, gap between both eyes 3 times as wide as diameter of eyes, with lens slightly distinct, dark, covering 20% of eye; posterior pair of eyes rounded, 1.2 times as wide as basal diameter of antennae, with lens barely distinct, dark, placed in middle of eye and covering 15% of it. aPodous anterior segment. Segment 2.5 times wider than long, 1.6 times as long as chaetiger 1, with flattening anterior margin, dorsum without marked transverse wrinkle. tentaCular Cirri. Slender, multi-articulated (Fig. 1B); postero-dorsal cirri extending posteriorly to chaetiger 5, twice as long as antero-dorsal cirri; antero-dorsal cirri extending posteriorly to chaetiger 2; postero-ventral cirri extended over opposite side of prostomium; antero-ventral cirri as long as posteroventral cirri and extending beyond 1.3 times length of palpophore; dorsal cirrophores cylindrical, ventral cirrophores ring-shaped, postero-dorsal cirrophores as long as antero-dorsal cirrophores, antero-ventral cirrophores twice as wide as postero-ventral cirrophores.
Pharynx. Non-everted, previously dissected. Jaws not dissected, with 10 well-developed and blunt denticles; pulp cavity with two canals. Brownish paragnaths on maxillary and oral rings (Fig. 1C), except reddish on areas VII-VIII, consisting of conical and p-bars; merged paragnaths and plate-like basements absent. Area I: 8, irregular patch of uneven cones, distal ones smaller; areas IIa: 18 and IIb: 22, three slightly regular rows of uneven cones in eyebrow-shaped patch, cones in outer-most row larger (Fig. 1C); area III: 29, four irregular rows of uneven cones in sub-rectangular patch, without distinct laterally-isolated cones, distal cones smaller (Fig. 1C); areas IVa: 35 and IVb: 37, pear-shaped patch with proximal half consisting of four irregular rows and distal half with three slightly regular rows of uneven cones (Fig. 1C); area V: 4, two oblique rows of two small, worn, conical paragnaths, near but placed behind level of paragnaths on area VI (Fig. 1C); areas VIa: 5 and VIb: 7, one transverse row of distal, uneven p-bars becoming shorter outwards (two in VIa, four in VIb; Fig. 1C-D) and one irregular patch of proximal, even cones (two in VIa and VIb; Fig. 1C-D); areas VII-VIII: 51, two bands of p-bars ventrally well separated, with anterior and posterior bands consisting each of two transversely aligned rows, furrow row and ridge row with one p-bar on each region (Fig. 1C). Areas VI-V-VI ridge pattern, λ-shaped. Gap between area VI and areas VII-VIII narrow, as wide as palpostyle.

Remarks
Neanthes chilkaensis was not compared with other species in the original description. Among all the currently known species of Neanthes, N. chilkaensis resembles N. indica brunnea and N. mossambica (Day, 1957), both from Mozambique, and N. talehsapensis (Fauvel, 1932) from the Songkhla Lake in Thailand. These species share a proximal region of dorsal ligule of similar size throughout the body (or slightly enlarged in posterior parapodia), presence of neuropodial postchaetal and notopodial prechaetal lobes at least in some anterior chaetigers, absence of neuropodial superior lobes, areas VII-VIII with two well-defined bands of more than 20 paragnaths, and area I with two or more paragnaths (see Villalobos-Guerrero & Idris 2021: table 2). (Southern, 1921). Lectotype (NHMUK 1938.5.7.27)  Nonetheless, N. chilkaensis is distinguishable from the species mentioned above according to the following diagnostic features: (I) the presence of p-bars on area VI, in contrast to its absence in those species; (II) the presence of solely p-bars on areas VII-VIII, in comparison to either conical only or both p-bars and conical ones in those species; (III) the multi-articulated tentacular cirri, in contrast to smooth ones in those species; (IV) four paragnaths on the area V of the pharynx, in comparison to none in those species; (V) a higher number of paragnaths on area III (29), in contrast to a lower number in N. indica brunnea (5-9) and N. talehsapensis (14); (VI) 5-7 paragnaths on area VI of the pharynx, in comparison to one in N. mossambica; (VII) the presence of oesophageal caeca, in contrast to its absence in N. mossambica; (VIII) aciculae mostly black, in contrast to pale coloured ones in N. talehsapensis; (IX) the notopodial prechaetal lobe distinctly smaller than median ligule throughout, in comparison to a subequal in anterior chaetigers of N. talehsapensis; (X) the dorsal cirri extending markedly beyond distal region of dorsal ligule, in contrast to those not extending beyond it in N. indica brunnea; (XI) the presence of a neuropodial postchaetal lobe throughout, in comparison to it being present in anterior chaetigers only in N. indica brunnea; (XII) the blade of heterogomph falcigers with blunt terminal tooth and inconspicuous tendon in middle and posterior parapodia, in contrast to hammer-headed terminal tooth and distinct tendon in N. indica brunnea; (XIII) the antennae well separated from each other, in comparison to those closely together in N. indica brunnea; and finally (XIV) the dorsum of segments with pigmentation bands, in contrast to it being present only in apodous segment in N. indica brunnea.

Fig. 1. Neanthes chilkaensis
The species was described and illustrated in detail by Southern (1921) and placed within the genus Nereis using specimens in immature and reproductive stages collected from the southern half of the Chilika Lake, India. The original description is a combination of the available specimens' morphology, which consisted of four epitokous (two females and males) specimens and seemingly four atokous specimens with different lengths and numbers of chaetigers: 92 mm, 81 chaet.; 65 mm, 84 chaet.; 56 mm, 66 chaet.; 44 mm, 81 chaet. However, the atokous specimen examined here does not match the size and number of chaetigers stated originally for those specimens. Southern (1921) used the specimen with 56 mm and 66 chaetigers to describe and illustrate the structures associated with the parapodia 1, 10, 50 and 60, which seems to correspond with one of two syntypes showing traces of desiccation deposited at the Zoological Survey of India (ZEV6281/7; http://www.zsicollections.in/search/ZSI0000004205) but not available for the present study. In general terms, the syntype examined matches the original description of N. chilkaensis. Hence, in order to fix the species definition (ICZN 1999, Art. 74.7.3), the specimen NHMUK 1938.5.7.27 is here selected as lectotype (ICZN 1999, Art. 74.1, 74.7.1) and described in detail with illustrations (ICZN 1999, Art. 74.7.2).
Moreover, some variations among the species deserve discussion. Southern (1921) described N. chilkaensis without paragnaths on area V, which are present in the lectotype in two oblique rows of four conical paragnaths near but placed behind the level of paragnaths of area VI. According to Southern (1921), the blades of subacicular heterogomph falcigers can wear distally giving two appearances: (I) long, slender, with many teeth; and (II) small, thick, with few teeth. In the syntype, only the first blade type was found possibly because most of the neurochaetae are broken or missing; however, judging Southern's falciger illustrations, the presence of the second type of falciger, as occurs in other species of Neanthes (Villalobos-Guerrero & Idris 2021; this study), is not disregarded.
The species was transferred to Neanthes by Hartman (1959), most likely founded on her proposal of distinguishing Neanthes from Nereis by the absence of homogomph falcigers (present in the latter genus; Hartman 1940Hartman , 1954. The species has been recognised in Neanthes until nowadays (Fauchald 1972;Wilson 1984;Nesemann & Sharma 2007), sometimes incorrectly treated in the unaccepted subgenus Nereis (Neanthes) (Misra et al. 1987;Nageswara Rao 1995). The species was briefly characterised by Fauvel (1932Fauvel ( , 1953 from some specimens collected in the type locality and other sites in the east of India. Later, Misra et al. (1987) and Nageswara Rao (1995) did the same using specimens exclusively from the Chilika Lake, although the characterisations were very short and incomplete.
Neanthes chilkaensis has been recorded from several localities along the shores of India (Parulekar 1971;Sunder Raj & Sanjeeva Raj 1987;Nesemann & Sharma 2007;Markande et al. 2014) andSri Lanka (de Silva 1965), which were mostly based on Fauvel's (1932Fauvel's ( , 1940Fauvel's ( , 1953) studies from India. These records are considered valid, although re-assessment is needed, especially those from Sri Lanka and the south and west of India.
antennae. Tapered, slender, short, extending forwards to tip of palpophore ( Fig. 2B) and posteriorly to one-third length of prostomium; antennae close together, with gap one-quarter as wide as basal diameter of antennae.

Remarks
Among all the currently known species of Neanthes, N. galetae from Galeta Island, Panama, resembles N. dawydovi (Fauvel, 1937) from Vietnam, N. egregicirrata (Treadwell, 1924) from Antigua in Lesser Antilles, N. hondoensis Khlebovich, 1996 from Japan, N. maculata Wu, Sun & Yang, 1981from China, N. mexicana Fauchald, 1972 from the Mexican Pacific, N. papillosa (Day, 1963) from South Africa, and N. vandersandei (Horst, 1924) from Indonesia. These eight species share proximal region of the dorsal ligules of similar size across the body (or slightly enlarged in posterior parapodia), absence of notopodial prechaetal, neuropodial postchaetal and superior lobes and subacicular homogomph spinigers, areas VII-VIII with one well-defined band of fewer than 20 paragnaths, area III with less than 20 paragnaths, and the area I with no more than two paragnaths (see Villalobos-Guerrero & Idris 2021: table 2). Additionally, they are all similar as they possess a pear-shaped prostomium and ovoid palpophores.
Nonetheless, N. galetae is easily distinguishable from them all by the following diagnostic features: (I) rounded patch of 9-11 paragnaths on area VI, in contrast to transverse row of up to four paragnaths in N. dawydovi, N. egregicirrata, N. maculata, N. papillosa and N. vandersandei, one paragnath in N. mexicana, and none in N. hondoensis; (II) the paired eyes of similar size, in contrast to the anterior pair markedly larger than the posterior pair in N. mexicana and N. papillosa; (III) the ventral ligule welldeveloped throughout, in contrast to that markedly reduced in posterior parapodia of N. dawydovi; (IV) the dorsum of all segments with transverse pigmentations, in comparison to its absence in chaetiger 2 in N. dawydovi; and (V) dorsal cirri with regular size, in contrast to it being markedly elongated on parapodia 6 in N. egregicirrata.
Likewise, N. galetae can also be differentiated from N. vandersandei in the following features based on its recent redescription (Villalobos-Guerrero & Idris 2021): (I) the multi-articulated tentacular cirri, in contrast to those smooth in N. vandersandei; (II) the presence of merged paragnaths on area IV, in comparison to its absence in N. vandersandei; (III) the presence of paired oesophageal caeca, in contrast to its absence in N. vandersandei; (IV) the presence of heterogomph spinigers in a few anterior supracicular neurochaetae, in comparison to its absence throughout in N. vandersandei; and (V) the blade of heterogomph falcigers with incurved terminal tooth and distinct tendon in middle and posterior parapodia, in contrast to those with blunt terminal tooth and inconspicuous tendon throughout in N. vandersandei.
The species was described by Fauchald (1977) using eight immature specimens collected from the Panama side of the Caribbean Sea among red algae, although only six of those collected in the Acanthophora and Laurencia zones correspond to the type series. At present, only four types remain (holotype and three paratypes). The type material examined here partially matches the barely-illustrated original description, which was seemingly based on the holotype. Neanthes galetae was described with a higher number of paragnaths in the areas of the maxillary ring (I: 2; II: 20; III: 18; IV: 25) when compared with specimens from the type series (I: 0-1; II: 8-14; III: 5-12; IV: 11-17). The dorsal cirri were mentioned as attached basally throughout in the original description, although it is attached medially in the middle and posterior parapodia of the type series. The replacement of homogomph spinigers by heterogomph spinigers in supracicular neurochaetae of posteriormost parapodia stated in the description was not observed in the type material. However, the heterogomph spinigers were observed in a few anteriormost parapodia, which were replaced by heterogomph falcigers in the following parapodia.
Neanthes galetae was not characterised or redescribed previously until this study. The original description includes a short, implicit comparison with four taxa of Neanthes in having a few paragnaths on area VII-VIII: Neanthes agulhana (Day, 1963) from South Africa, N. dawydovi, N. kerguelensis (Mclntosh, 1885 from the Kerguelen Islands, and N. kerguelensis oligodonta (Augener, 1913) from Australia. They were distinguished by paragnaths occurring on area I, although variations overlap in the species (see Villalobos-Guerrero & Idris 2021; this study). The subspecies is not considered valid presently (Wilson 1984), and the differences with N. dawydovi were mentioned above. Neanthes galetae is easily distinguished from both N. agulhana and N. kerguelensis by the lack of notopodial prechaetal lobe (present in those species) and the rounded patch of 9-11 paragnaths on area VI (transverse row of up to 4 paragnaths in N. agulhana and 0-1 paragnath in N. kerguelensis). Neanthes galetae is also different from N. kerguelensis by the lack of neuropodial postchaetal lobe and subacicular homogomph spinigers (both present in N. kerguelensis).
The species has also been recorded from the Pacific part of El Salvador (Planas et al. 2013) and Colombia (Jaime et al. 1999) in ecological studies. However, they are considered doubtful until a re-examination of the voucher material is performed.
Prostomium. Campanulate, as long as wide (Fig. 3B); anterior end broad, distally complete; anterolateral gap beside palpophore narrow, as wide as antennal diameter; dorsal groove distinct, shallow, running mid-subdistally. Nuchal organs exposed, broad, twice as wide as diameter of posterior pair of eyes.
antennae. Tapered, thick, long, extending forwards beyond tip of palpophore and posteriorly to halflength of prostomium; antennae separated, with gap as wide as basal diameter of antennae (Fig. 3B).
eyes. Paired eyes purplish but gradually fading, arranged in a trapezoid form; gap between both pairs two-thirds as wide as diameter of posterior pair of eyes (Fig. 3B); anterior pair of eyes oval, 1.6 times as wide as basal diameter of antennae, gap between both eyes 3.5 times as wide as diameter of eyes, with lens barely distinct, translucent, covering 20% of eye; posterior pair of eyes rounded, 1.7 times as wide as basal diameter of antennae, with lens distinct, translucent, placed in middle of eye and covering 20% of it. aPodous anterior segment. Segment 4 times wider than long, 1.5 times as long as chaetiger 1, with rounded anterior margin and ventrolateral projections; dorsum without marked transverse wrinkle. tentaCular Cirri. Cirri of right flank dehiscent, remaining slightly thickened, smooth (Fig. 3B); posterodorsal cirri broken, extending posteriorly to chaetiger 6, but according to original description, extending to chaetiger 10; antero-dorsal cirri broken, extending posteriorly to chaetiger 3; postero-ventral cirri extended over opposite side of prostomium; antero-ventral cirri slightly longer than postero-ventral cirri and extending beyond twice length of palpophore; dorsal cirrophores cylindrical, ventral cirrophores ring-shaped, postero-dorsal cirrophores 1.5 times as long as antero-dorsal cirrophores, antero-ventral cirrophores 1.2 times as wide as postero-ventral cirrophores.
Pharynx. Everted, in poor condition, damaged, jaws and several areas missing, only areas V and VI remain. Pharynx of second syntype described here, everted, in poor condition, damaged, jaws missing; paragnaths on maxillary and oral rings amber, barely distinct but stained here (Fig. 3C-D), consisting of conical, p-bars, and merged paragnaths; plate-like basements absent. Area I: 1, small cone; areas IIa: 31 and IIb: 38, three slightly regular rows of uneven cones in eyebrow-shaped patch, subdistal cones larger, fang-shaped in outer-most row (Fig. 3C); area III: 18, four irregular rows of uneven cones in sub-oval patch, with three distinct laterally-isolated cones, distal cones smaller; areas IVa: missing and IVb: 26, L-shaped patch with proximal half consisting of two regular rows and distal half with four slightly regular rows of uneven cones and four long and slender merged paragnaths (3-4 times longer than wide) located near jaw; area V: 1 (1), small conical paragnath placed on same level between most distal and proximal paragnaths on area VI (Fig. 3C); areas VIa: 7 (5) and VIb: 6 (5), oblique transverse row of uneven p-bars becoming shorter outwards (six in VIa, four in VIb; Fig. 3C) and one single outermost cone (Fig. 3C); areas VII-VIII: 31, two bands of cones, with anterior band consisting of two transversely aligned rows (furrow row with one small cone and ridge row with one fang-shaped cone on each region) (Fig. 3D), and posterior band with two transverse rows (Fig. 3D). Areas VI-V-VI ridge pattern, π-shaped. Gap between area VI and areas VII-VIII narrow, as wide as palpostyle. dorsal ligule. Proximal region even throughout, except slightly humped from middle parapodia towards posterior end; shorter than distal region of dorsal ligule in anteriormost parapodia (Fig. 3E), as long as that in anterior, middle and posterior parapodia ( Fig. 3F-I), 1.5 times as long as that in posteriormost parapodia; two irregular glandular patches covering partially proximal region of dorsal ligule in anteriormost and anterior parapodia (Fig. 3E-F) and entirely covering that in following parapodia ( Fig. 3G-I). Distal region well developed, becoming slightly longer towards posterior end; digitiform in anteriormost parapodia (Fig. 3E), bluntly rounded in anterior parapodia (Fig. 3F), conical in middle parapodia (Fig. 3G-H), bluntly conical in remaining parapodia (Fig. 3I); as long as or slightly shorter than median ligule in anterior parapodia (Fig. 3F), longer than that in following parapodia; projecting beyond notoacicula throughout; one irregular glandular patch covering entirely distal region of dorsal ligule (Fig. 3F-I).
aCiCulae. Colour mostly faded by long-term preservation. Notoaciculae absent in first two chaetigers (Fig. 3E). Neuroaciculae extending similarly to distal end of notoaciculae throughout, with proximal half 1.3-1.5 times as wide as notoaciculae.
Prostomium. Campanulate, as long as wide or slightly longer than wide (Fig. 4B); anterior end broad, distally complete; anterolateral gap beside palpophore narrow, as wide as antennal diameter; dorsal groove distinct, shallow, running mid-subdistally. Nuchal organs deeply embedded, broad, 1.7-2 times as wide as diameter of posterior pair of eyes.
PalPoPhores. Sub-conical, as wide as long to slightly wider as long (Fig. 4B), subequal to entire length of prostomium; with distinct sub-distal transverse groove. Palpostyles ovoid, thick, with diameter as wide as two-fifths to half of palpophore.
antennae. Tapered, thick, long, extending forwards beyond tip of palpophore and posteriorly to half to two-fifths length of prostomium; antennae separated, with gap subequal to three-quarters as wide as basal diameter of antennae (Fig. 4B).
Pharynx. Non-everted. Jaws with distal quarter brownish, remaining yellow amber, with 9-10 welldeveloped and sharp denticles (Fig. 4E); pulp cavity with two canals (Fig. 4F). Reddish-brown paragnaths on maxillary and oral rings (Fig. 4G-H), consisting of cones, p-bars, and merged paragnaths; plate-like basements absent. Area I: 1-2, cones; areas IIa: 20-27 and IIb: 19-27, two slightly regular rows of uneven cones in eyebrow-shaped patch, subdistal cones larger, fang-shaped in outer-most row (Fig. 4G); area III: 18-28, two to four irregular rows of uneven cones in sub-oval or sub-rectangular patch, with two to four distinct laterally-isolated cones, distal cones smaller; areas IVa: 24-45 and IVb: 26-48, L-shaped patch (Fig. 4G) with proximal half consisting of two regular rows and distal half with three or four slightly regular rows of uneven cones and four to six long and slender (3-4 times longer than wide) merged paragnaths located near jaw; area V: 1 (1), coarse conical paragnath placed slightly behind level of paragnaths on area VI (Fig. 4G); areas VIa: 5-8 and VIb: 5-7, oblique transverse row of uneven p-bars becoming shorter outwards (four to seven in VIa, four to six in VIb; Fig. 4H) and one single outermost cone (Fig. 4H); areas VII-VIII: 28-33, two bands of cones ventrally well separated, with anterior band consisting of two transversely aligned rows (furrow and ridge rows with one coarse cone on each region), and posterior band with two transverse slightly displaced rows (Fig. 4G). Areas VI-V-VI ridge pattern, π-shaped. Gap between area VI and areas VII-VIII narrow, as wide as palpostyle.
neuroPodial inferior lobe. Slightly developed and longer than neuroacicular ligule in first four or five parapodia, absent in following chaetigers.

Remarks
Neanthes helenae was not compared in the original description by Kinberg (1865) Wu, Sun & Yang, 1981, N. indica brunnea, N. nubila (Savigny, 1822 likely from the North Atlantic (precise locality unknown), and N. talehsapensis (Fauvel, 1932) from the Gulf of Thailand. These species share proximal region of dorsal ligules of similar size throughout the body (or slightly enlarged in posterior parapodia), presence of neuropodial postchaetal and notopodial prechaetal lobes at least in some anterior chaetigers, absence of neuropodial superior lobes, areas VII-VIII with two well-defined bands of more than 20 paragnaths, and area I with no more than two paragnaths (see Villalobos-Guerrero & Idris 2021: table 2). Kinberg, 1865. A-C, I-P. Holotype of Nereis (Neanthes) nanciae Day, 1949(NHMUK 1950 Day, 1949(NHMUK 1950  Neanthes helenae was described without illustrations by Kinberg (1865) using an unknown number of specimens collected near St Helena Island. The type material was not addressed in the literature, and most of its morphology remained unknown until this study. The two syntypes are in poor condition, but the relevant diagnostic features are still visible, and thus are useful to define the species' morphology. Day (1949) described Nereis (Neanthes) nanciae from St Helena Island with several atoke specimens, but he did not justify the proposal as a new species. Later, Hartman (1959) transferred the species to Neanthes and suggested it as a possible synonym of the incompletely known N. helenae. Fauchald (1972) considered Neanthes nanciae as a valid species, but Wilson (1984) agreed with Hartman's assumptions, suggesting N. nanciae as a possible junior synonym of N. helenae. After the detailed examination of both species' type material, N. helenae is here regarded as the senior synonym of N. nanciae as a result of the overlap in unique and diagnostic features among other species in Neanthes, such as the presence of p-bar paragnaths on area VI, ventrolateral projections on the apodous segment and the presence of simple chaetae (fused falcigers) in supracicular fascicle.

Distribution
Saint Helena Island.

Ecology
The upper part of the intertidal zone to 73 m depth (Day 1949).
Prostomium. Campanulate, 1.4 times longer than wide (Fig. 5B); anterior end broad, distally complete; anterolateral gap beside palpophore broad, 1.5 times as wide as antennal diameter; dorsal groove distinct, shallow, running through distal three-quarters of prostomium. Nuchal organs deeply embedded, medium size, subequal to diameter of posterior pair of eyes.
PalPoPhores. Sub-conical, longer than wide, as long as four-fifths of entire prostomium (Fig. 5A); with distinct sub-distal transverse groove. Palpostyles ovoid, thin, with diameter as wide as one-fifth of palpophore.
antennae. Tapered, slender, long, extending forwards slightly beyond end of palpophore and posteriorly to half-length of prostomium; antennae well separated, with gap as wide as basal diameter of antennae.
eyes. Paired eyes blackish, arranged in a trapezoid form (Fig. 5B); gap between both pairs one-third as wide as diameter of posterior pair of eyes; anterior pair of eyes sub-rounded, 1.2 times as wide as basal diameter of antennae, gap between both eyes 3.5 times as wide as diameter of eyes, with lens slightly distinct, whitish, covering 10% of eye; posterior pair of eyes rounded, as wide as basal diameter of antennae, with lens barely distinct, dark, placed in middle of eye and covering 15% of it. aPodous anterior segment. Segment 3.5 times wider than long, 1.4 times as long as chaetiger 1, with rounded anterior margin (Fig. 5B), dorsum without marked transverse wrinkle. tentaCular Cirri. Slender, smooth (Fig. 5B); postero-dorsal cirri extending posteriorly to chaetiger 8 (4), twice as long as antero-dorsal cirri; antero-dorsal cirri extending posteriorly to chaetiger 4 (2); postero-ventral cirri extended over opposite side of prostomium; antero-ventral cirri nearly as long as postero-ventral cirri and extending slightly beyond palpophore; dorsal cirrophores cylindrical, ventral cirrophores ring-shaped, postero-dorsal cirrophores 1.5 times as long as antero-dorsal cirrophores, antero-ventral cirrophores 1.5 times as wide as postero-ventral cirrophores.
Pharynx. Non-everted, previously dissected. Jaws reddish in distal quarter, remaining amber, with 8 (5) blunt, wear denticles; pulp cavity with two canals. Brownish conical paragnaths on maxillary and oral rings (Fig. 5C), except reddish in areas VII-VIII; merged paragnaths and plate-like basements absent. Area I: 6 (2), triangular patch of uneven cones, single distal cone smaller; areas IIa: 15 (15) and IIb: 15 (17), three irregular rows of uneven cones in sub-triangular patch, cones in outer-most row larger (Fig. 5C); area III: 32 (23), four slightly regular rows of uneven cones in rectangular patch, without distinct laterally-isolated cones, distal cones smaller (Fig. 5C); areas IVa: 31 (25) and IVb: 32 (24), bow-shaped patch with proximal half consisting of four irregular rows and distal half with three irregular rows of uneven cones (Fig. 5C); area V: 0 (0) (Fig. 5C); areas VIa: 1 (1) and VIb: 1 (1), coarse cone, distally wear in VIa (Fig. 5C-D); areas VII-VIII: 50 (42), two well-separated bands of cones, with anterior band consisting of two transversely slightly displaced rows of coarse cones (furrow row and ridge row with one cone on each region) (Fig. 5C), and posterior band with three transverse rows slightly displaced from each other (furrow row middle with one cone on each region, distal ridge row with two or three coarse cones on each region, proximal ridge row with one small cone on each region) (Fig. 5C). Areas VI-V-VI ridge pattern, λ-shaped. Gap between area VI and areas VII-VIII narrow, as wide as distal end of palpophore.
dorsal ligule. Proximal region even towards posterior end (Fig. 5E-I); shorter than distal region of dorsal ligule in anteriormost and anterior parapodia ( Fig. 5E-F), as long as that in following parapodia ( Fig. 5G-I); glandular patches absent. Distal region well developed with similar length throughout ( Fig. 5E-I); subulate, slender, as long as median ligule in anterior parapodia, conical and smaller than that in following parapodia; projecting beyond notoacicula throughout; glandular patches absent.
Ventral Cirri. Cirriform, slender; two-thirds as long as ventral ligule in anteriormost and anterior parapodia (Fig. 5E-F), one-third as long as that in middle parapodia, levelling base of that in posterior parapodia.
Pygidium. In regeneration, with anal cirri as long as last 6 chaetigers; cirrophores of anal cirri barely developed.

Remarks
Among all the currently know species of Neanthes, N. mossambica from Mozambique resembles N. chilkaensis from India, N. indica brunnea from Mozambique, and N. talehsapensis from the Gulf of Thailand. These species share proximal region of dorsal ligule of similar size throughout the body (or slightly enlarged in posterior parapodia), presence of neuropodial postchaetal and notopodial prechaetal lobes at least in some anterior chaetigers, absence of neuropodial superior lobes, areas VII-VIII with two well-defined bands of more than 20 paragnaths, and area I with two or more paragnaths (see Villalobos-Guerrero & Idris 2021: table 2).
Nonetheless, N. mossambica is distinguishable from them all by the following diagnostic features: (I) one paragnath on area VI, in contrast to 5-7 in N. chilkaensis, 4-6 in N. indica brunnea, 4-5 in  (Day, 1957). Holotype (NHMUK 1961.16.22)  N. talehsapensis; (II) the absence of oesophageal caeca, in contrast to its presence in N. chilkaensis and N. indica brunnea; (III) the area VI with conical paragnaths only, in contrast to both conical and p-bars in N. chilkaensis; (IV) the smooth tentacular cirri, in contrast to multi-articulated in N. chilkaensis; (V) the area V without paragnaths, in comparison to their presence in N. chilkaensis; (VI) the area VII-VIII with conical paragnaths only, in comparison to p-bars only in N. chilkaensis; (VII) the prostomium longer than wide, in contrast to that as long as wide in N. talehsapensis; (VIII) the aciculae mostly black, in contrast to those rather pale colour in N. talehsapensis; (IX) the notopodial prechaetal lobes distinctly smaller than median ligule throughout, in contrast to those that are subequal in anterior chaetigers of N. talehsapensis; (X) 23-32 paragnaths on area III, in comparison to 5-9 in N. indica brunnea; (XI) the dorsal cirri extending markedly beyond distal region of dorsal ligule, in contrast to those not extending beyond in N. indica brunnea; (XII) the presence of neuropodial postchaetal lobe throughout, in contrast to that in anterior chaetigers only in N. indica brunnea; (XIII) the heterogomph falciger blades with blunt terminal tooth and inconspicuous tendon in middle and posterior parapodia, in contrast to those with hammer-headed terminal tooth and distinct tendon in N. indica brunnea; (XIV) the antennae well separated from each other, in comparison to those closely together in N. indica brunnea; and (XV) the dorsum of segments with pigmentation bands, in contrast to a pigment band present only in the apodous segment of N. indica brunnea.
Neanthes mossambica was briefly described, scarcely illustrated and placed within the subgenus Nereis (Neanthes) by Day (1957) using two atokous specimens collected from the Morrumbene Estuary, Mozambique. Both the holotype and paratype match the original description. The species has always been considered within Neanthes, either recognized at the subgeneric (Hartman 1959;Day 1967;Raghunath 1976) or generic (Pillai 1965;Fauchald 1972;Wilson 1984) level, except Day (1974) who for an unknown reason enlisted the species as Nereis mossambica. The combination Neanthes mossambica that prevails nowadays was used for the first time by Pillai (1965) while comparing his new species Neanthes manatensis from Manat, Philippines. Both species are morphologically similar, however, as Pillai (1965) aptly mentioned, they can be distinguished by the number of paragnaths on the area I, and also on areas III and VII-VIII.
Neanthes mossambica was briefly characterised by Day (1967) but based solely on the original description. No more attempt to describe the morphology of the species was performed. The original description includes a short, implicit comparison with three other Neanthes species (formerly in Nereis): Neanthes chilkaensis, N. talehsapensis, and N. glandicincta (Southern, 1921) from near Calcutta, India. The first two species were distinguished above in detail from N. mossambica. Neanthes glandicincta was recently redescribed using a syntype and specimens from Myanmar and Singapore (Lee & Glasby 2015) and characterised with individuals from Malaysia and Thailand (Ibrahim et al. 2019;Azmi et al. 2021). Neanthes mossambica can be easily distinguished from N. glandicincta by the absence of neuropodial superior lobe (present in N. glandicincta), the presence of two bands of paragnaths on area VII-VIII (one or none bands in N. glandicincta), the presence of heterogomph falcigers throughout the body (absent in anterior parapodia in N. glandicincta), among other features.
The species has been recorded in India (Raghunath 1976) and Bangladesh (Muir & Hossain 2014), although these records are questionable since no species' characterisations were provided.

Reproduction
Unknown.

Discussion
Four species of Neanthes, N. chilkaensis, N. galetae, N. helenae and N. mossambica, are redescribed. Diagnostic features are illustrated, and habitat, distribution and taxonomic remarks are given for the species.
Redescriptions based on the type material permit to recognize species as valid. These four species were poorly or not distinguished from others in the original descriptions. The current study provides sufficient evidence to warrant their separation from other similar congeners. Detailed comparisons of the species using a combination of former and recent diagnostic characters enabled reliable differentiation. For instance, the typical characters related to the paragnaths and parapodial structures (Hartman 1954;Hutchings & Turvey 1982;Wilson 1984;Wu et al. 1985;Khlebovich 1996;Bakken 2002Bakken , 2006Glasby et al. 2011;Hsueh 2019a;Ibrahim et al. 2019), combined with the latest proposed ridge patterns on areas VI-V-VI and the occurrence of oesophageal caeca allow their distinction among Neanthes species.
Additionally, redescriptions also allow one to complete the species' description, delimit their morphology, and detect previously-overlooked or unique features. In this study, the presence of pointed bar-shaped paragnaths ('p-bars') on area VI of N. chilkaensis and N. helenae, as well as its sole presence on the areas VII-VIII in N. chilkaensis, is detected for the first time in the family Nereididae. The p-bar is a relatively recent paragnath term defined as a small bar with a protruding apex in one end (Bakken et al. 2009). It was first introduced for some species of Pseudonereis, which bear that type of paragnath on areas II, III, IV and VII-VIII (Bakken 2007). Later, the p-bars were also recorded on the oral ring of some species of Alitta (Villalobos-Guerrero & Carrera-Parra 2015), Nereis (Conde-Vela & Salazar-Vallejo 2015), and presently in Neanthes. Nonetheless, detailed studies on the genera are needed since the p-bars might have been described as conical paragnaths in earlier literature.
Moreover, N. helenae is unique within the genus by showing ventrolateral projections on the apodous segment. These are similar to the flap-like projections present in the species of Cheilonereis Benham, 1916 andLaevispinereis He &Wu, 1989. The development of projections is dependent on the size of the specimens, as it becomes more expanded in larger individuals of Cheilonereis (Harrington 1897;Ramsay 1914). A similar pattern was observed in N. helenae, although the flap is less developed (Fig. 4C-D). Cheilonereis and Laevispinereis were mainly diagnosed by the presence of this structure, although other features in the parapodial lobes, ligules and chaetae were also found relevant to define their generic morphology (sensu Santos et al. 2005). Neanthes helenae is still retained in Neanthes as it resembles more to the type species, N. vaalii, than to C. cyclurus (Harrington, 1897) and L. fujianensis He & Wu, 1989. For instance, N. helenae is similar to N. vaalii by having paragnaths on the oral ring, heterogomph falcigers in neuropodia, rounded and short neuropodial superior and inferior lobes and by lacking notopodial prechaetal and neuropodial postchaetal lobes, in comparison to L. fujianensis that has papillae on the oral ring, homogomph falcigers in neuropodia, conical and long notopodial and neuropodial lobes (He & Wu 1989). Also, N. helenae is similar to N. vaalii by having spinigers only in notopodia and dorsal ligule of similar size throughout the body, in contrast to falcigers present in notopodia and dorsal ligule expanded in posterior notopodia in C. cyclurus. However, the re-allocation of N. helenae in another genus is not disregarded after the revision of Neanthes is developed considering its polyphyletic status.
Further descriptive work is necessary for a complete survey of the characters in Neanthes. A better understanding of the morphology of characters and their potential in phylogenetic analyses are warranted for the urgent revision of the genus. Currently-common characters often not included in the original descriptions or overlooked in the redescriptions should be considered, such as articulations on tentacular cirri, merged paragnaths on area IV, and laterally isolated paragnaths on area III, among others. Likewise, particular attention should be paid to those recently proposed, as species beyond the four redescribed here and other four elsewhere (see Villalobos-Guerrero & Idris 2021) remain mostly unknown.